The present invention relates in general to electronic circuits and components therefor, and is particularly directed to a new and improved constant current generator circuit-based arrangement for providing a programmable bi-directional voltage offset for a pulse width modulation controller (PWM) of a PWM-based DC-DC converter.
FIG. 1 diagrammatically illustrates the configuration of a conventional pulse width modulation (PWM) controlled DC-DC power converter. As shown therein a voltage terminal 10 is coupled to receive a reference voltage V1, such as that supplied by a digital-to-analog converter (DAC). This reference voltage is coupled to a reference terminal pin REF through a filter 11, such as that comprised of a resistor R4 and capacitor C3. The voltage terminal REF, in turn, is coupled to the non-inverting (+) input 21 of an operational amplifier (OP AMP) 20. The output 23 of the OP AMP is coupled to a PWM generator circuit 30, the output 33 of which is coupled through an inductor (L1) 40 to an output node 50.
The PWM generator circuit""s output 33 is fed back as an output VOUT to the inverting (xe2x88x92) input 22 of amplifier 20 through a filter 60 containing a series resistor R2 coupled to a feedback path FB to the output 23 of the amplifier 20, and through a series connection of a capacitor Cl and a resistor R1 to a node COMP, that receives an amplified and filtered error signal from the amplifier 20. The PWM generator circuit 30 drives a load (represented by a resistor R3) coupled to the output node 50. The COMP terminal controls the PWM generator in a direction to minimize the difference between the voltage REF and the voltage VOUT.
PWM-based DC power supply integrated circuits are often required to have a user adjustable offset. Specifically, in some applications it may be desirable to have the output voltage generated at the output node VOUT different from the input reference voltage V1 by a prescribed (e.g., user-programmable) offset value. Depending upon the application, it may be necessary that this offset voltage be bi-directional, and must be repeatable and stable. Generating this offset should require a minimum number of external components, should not be dependent on other factors such as power supply voltage and, for economy of packaging, should require the least number of integrated circuit pins. In addition, to alleviate the design of supporting components, the offset should not require a stable current reference, but only a stable voltage reference (such as a bandgap voltage reference).
In accordance with the present invention, these objectives are successively achieved by a constant current generator circuit that is configured to allow external programming of either a positive (+) or a negative (xe2x88x92) polarity output current, that is readily injected into one of two locations of the PWM controller circuit of a DC-DC voltage converter. As will be described, the parameters of the DC-DC converter""s offset voltage will depend upon the connection of a single programming pin to one of two programming resistors. The programming resistors are respectively referenced to different supply rail voltages (VCC and VSS), on the one hand, and will also depend upon where, within the PWM-controlled DC-DC converter, the programmed constant current is injected.
The user-programmable, bi-directional constant current generator contains first and second operational amplifiers that drive associated complementary switching devices (such as PMOSFET and NMOSFET switches). The input of one of the amplifiers is coupled to a first DC voltage referenced to a first DC power supply rail (e.g., VCC). The input of the other amplifier is coupled to a second DC voltage referenced to another supply rail (e.g., VSS or ground). The current flow paths through the two switching devices are coupled in series between an offset input terminal to which a programming input pin is coupled and an offset current output terminal.
The first programming resistor is coupled between the VCC supply rail and a first programming pin; the second programming resistor is coupled between the VSS supply rail and a second programming pin. By selectively connecting the programming input pin to one of the first and second programming pins (and thereby to their associated resistors), the bi-directional constant current generator is user-programmed to supply one of a positive polarity and negative polarity offset current to the offset current output terminal.
In a first, xe2x80x98sourcingxe2x80x99 current programming mode, the first programming pin is coupled to the programming input pin, while the second programming pin is open. As a result, that one of the amplifiers which is coupled to the first programming resistor, will cause its associated switching device to conduct. With its associate switching device conducting, that amplifier attempts to drive the voltage at its input port to match that amplifier""s reference voltage. At the same time, the other amplifier maintains its associated switching device in the off state. With the one amplifiers"" switching device turned on, a sourcing current will flow from the supply rail through the first programming resistor, the single programming terminal and the current path of the turned-on switch to the current source""s output terminal. The magnitude of this current is effectively equal to the reference voltage divided by its associated series resistor.
In a second, xe2x80x98sinkingxe2x80x99 current programming mode, the second programming pin is coupled to the programming input pin, while the first programming pin is open. As a result, the second amplifier will cause its associated switching device to conduct, while the one amplifier maintains its switching device in the off state. The second amplifier attempts to make voltage at the input port match that amplifier""s reference voltage. With the second amplifiers"" switching device turned on, a sinking current will flow from a supply rail through the second programming resistor, the single programming terminal and the current flow path of the second amplifier""s associated turned-on switch to the output terminal. The magnitude of the sinking current is effectively equal to the reference voltage divider by its associated series resistor.
A selected one of these xe2x80x98sourcingxe2x80x99 and xe2x80x98sinkingxe2x80x99 currents as generated by the user-programmable, bi-directional constant current generator of the invention is connectable to either of two locations in the PWM controller of the PWM switching DC power supply to provide a constant offset voltage. The reference voltages employed within the constant current source may be based upon a bandgap voltage (referenced to VSS or ground (GND)). In a non-limiting, but preferred embodiment, the first reference voltage (referenced to VCC) may be derived from the second voltage (referenced to VSS)